A high-speed non-volatile optical memory is very important for an optical network. High-speed data processing such as receiving, storing and resending data is a main function of an optical network server. High-speed optical memory is required to realize such a function at high speed.
As disclosed in U.S. Pat. No. 5,740,117, a method of storing optical signals in a loop structure in order to store a fixed amount of information within an optical loop is known.
Further, as disclosed in U.S. Pat. Nos. 5,999,284 and 6,647,163, an optical memory device with a Mach-Zender interferometer which has a semiconductor optical amplifier as a component, is known. A major restriction with regards to the operational speed of such a device, results from a long intersubband transition time of electrons in a semiconductor. In addition, such a memory is volatile, so that it is disadvantageous that data cannot be stored for a long time.
Moreover, as disclosed in U.S. Pat. No. 7,171,096, which is a patent family of Japanese Laid Open Patent No. 2006-018964, anon-volatile high-speed optical memory element is known. A main advantage of such a memory element is a very high operation speed.
FIG. 1 shows a schematic diagram illustrates an operational principal of a conventional optical memory element, which is disclosed in U.S. Pat. No. 7,171,096 (U.S. patent application Ser. No. 11/172,861) and Japanese Laid Open Patent No. 2006-018964. The memory element is made up of a semiconductor photodiode with electrode made of ferromagnetic metal.
The information data is stored according to the magnetization directions of the ferromagnetic metal electrode. The information data is recorded by circularly polarized light. When reversed voltage is applied to the photo diode, circularly polarized light excites spin polarized current in the photodiode. The spin polarized current is injected into the ferromagnetic metal electrode thereby reversing the magnetization.
As shown in FIG. 1 the memory is made up of two major components, a semiconductor region and a single-domain ferromagnetic layer.
Data is stored according to magnetization directions of the ferromagnetic metal layer.
The optical pulse excites photo electrons in the semiconductor, and the photo electrons are injected into the ferromagnetic metal layer by applying voltage. If the light is circularly polarized, or elliptically-polarized, the excited photo electrons generated in semiconductor region are spin-polarized. That means that the number of spins in an upward direction is different from that in a downward direction.
When the spin-polarized electrons are injected into the ferromagnetic metal layer, the magnetic torque is generated, which is capable of reversing the magnetization of the ferromagnetic metal (J. C. Slonczewski, Journal of Magnetism and Magnetic Materials, Vol. 159, pp. L1-L7, 1996; and J. Z. Sun, Physical Review B, Vol. 62, pp. 570-578, July 2000). Therefore, optical information data expressed by difference of polarization states can be stored as magnetization directions in the ferromagnetic layer.
In addition, in these references, a high-speed demultiplexing method is disclosed.
Both an optical pulse for data and an optical pulse for clock are simultaneously emitted on the memory. The polarization of the optical pulse for data and that for clock is linear polarization and is orthogonal to each other. Since the polarization of the synthesized optical pulse becomes circular polarization only where these optical pulses are simultaneously emitted, the information data is recorded in the memory through excitation of spin-polarized electrons.
Since other optical pulses which are not matched each other at time axis, are linearly polarized, spin-polarized current is not generated, so that data is not memorized in the memory.
In order to read out stored data, the magneto-optical effect is used. Magnetization information in the free layer is read by using Faraday effect or effect of non-reciprocal loss, by illumination of this layer by the light.
In order to reverse the magnetization of the ferromagnetic metal layer with a realistic injection current, the volume of the ferromagnetic metal layer should be relatively small.
In case where injected current is smaller than 10 mA, the thickness of ferromagnetic metal layer should be approximately 2-5 nm, and the area should be approximately 0.02 μm2 (See Kubota et al. Japanese Journal of Applied Physics Vol. 44, pp. L1237-L1240, 2005).
Since magneto-optical effect is small when the volume of material is small, it is difficult to read out information data stored in the ferromagnetic metal layer. In such a situation, application of this memory is difficult.